Start with adding the following API Declarations to the Declarations section of a module:
Declare Function FindWindow Lib "user32" Alias _
"FindWindowA" (ByVal lpClassName As String, _
ByVal lpWindowName As String) As Long
Declare Function PostMessage Lib "user32" Alias _
"PostMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, _
ByVal wParam As Long, lParam As Any) As Long
Public Const WM_CLOSE = &H10
And here's the code. Just replace apptitle with the exact title of the application you want to close (i.e. "Calculator"

:
Dim winHwnd As Long
Dim RetVal As Long
winHwnd = FindWindow(vbNullString, "apptitle"
If winHwnd <> 0 Then
RetVal = PostMessage(winHwnd, WM_CLOSE, 0&, 0&)
If RetVal = 0 Then
MsgBox "Error posting message."
End If
Else
MsgBox "Application is not open."
End If